fix: only apply merging to plain objects

[jaredrethman]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

When merging caches using updateCacheKey the logic assumes all values are plain objects. This is causing Array cache types being converted to objects.

Example:

['one', 'two']
// is becoming
{"0":"one", "1": "two"}

This PR aims to fix this.

How to test the changes in this Pull Request:

1. Checkout and build assets git checkout origin/fix/wizardapifetch-cache-merging && npm run build.
2. Navigate to /wp-admin/admin.php?page=newspack-settings
3. Scroll down to "Webhook Endpoints" and add, edit and delete Endpoints and confirm changes reflect between tab changes and page refreshes.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[miguelpeixe]

Fixes the issue!